Method of effecting cyclic reversible holographic recording in a thermo plastic material

ABSTRACT

A method of effecting cyclic, reversible, holographic recording in a thermoplastic material including the steps of sensitizing the thermoplastic material by placing the same by a corona charge at a positive or negative potential and thereafter performing a holographic exposure and developing and fixing by cooling the resultant hologram. By increasing the number of recording cycles, the corona discharge voltage required for sensitizing is increased and the thermoplastic material is therefore placed at progressively higher positive or negative potentials.

United Stats-1 a V/ METHOD OF EFFECTING CYCLIC,

REVERSIBLE, HOLOGRAPHIC RECORDING IN A THERMO-PLASTIC MATERIAL [75]Inventor: Joachim Ost, Martinried, Germany [73] Assignee: SiemensAktiengesellschaft, Berlin and Munich, Germany [22] Filed: May 16, 1973[21] Appl. No.: 360,768

[ll] 3,821,722 June 28, 1974 3.245.053 4/1966 Brown 340/173 TP PrimaryExaminer-Terrell W. Fears Attorney, Agent, or Firm--Hill, Gross,Simpson, Van Santen. Steadman. Chiara & Simpson 5 7 1 ABSTRACT A methodof effecting cyclic, reversible, holographic recording in athermoplastic material including the steps of sensitizing thethermoplastic material by placing the same by a corona charge at apositive or negative potential and thereafter performing a holographicexposure and developing and fixing by cooling the resultant hologram. Byincreasing the number of recording cycles, the corona discharge voltagerequired for sensitizing is increased and the thermoplastic material istherefore placed at progressively higher positive or negativepotentials.

1 Claim, 1 Drawing Figure OR IN: sac/ware METHOD OF EFFECTING CYCLIC,REVERSIBLE, I-IOLOGRAPHIC RECORDING IN A THERMO-PLASTIC MATERIALBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method for effecting cyclic, reversible, holographicrecording in a thermoplastic material, and is more particularlyconcerned with such holographic recording in which, prior to each newrecording cycle, the thermoplastic material is placed adjacent a coronacharge at a positive or negative potential and thus sensitized.Afterward holographic exposure is carried out and the resultant hologramis developed by heating and fixed by cooling.

2. Description of the Prior Art Holographic recording techniques inwhich a photochemical process is not required are already well known inthe art. For example, reference may be taken to the book by Kiemle andRoss entitled Einfuhrung in die Technik der Holographie, AkademischeVerlagsgesellschaft, Frankfurt am Main, 1969, Page 217. The storagemedium employed in this type of process is a thermoplastic syntheticmaterial which has been applied to a layer of transparent,photoconducting material on an electrically conductive, clear, specialglass substrate. To effect sensitizing, the external surface of thesynthetic material is provided with a uniformly distributed, positive ornegative charge by the use of a corona charging mechanism whichinfluences a negative or positive charge at the glass-photoconductorinterface. At the time of exposure, the photoconductor becomesconductive at points where light is incident, whereas at unexposedlocations it continues to insulate. At the conductive locations, thecharges pass through the photoconductor up to the interface with thesynthetic material layer. After exposure, the arrangement is renderedinsensitive to light by a secondary charge.

In order to develop the hologram, the synthetic material is heated up tothe softening point. In so doing, locations carrying positive chargesare attracted by the opposite negative charges, and vice versa, so thata surface relief formation is produced corresponding to the chargedistribution and therefore to the exposure, and this surface relief isfixed by cooling. In this manner, a surface phase hologram is produced.

The hologram can be erased again if the layer is heated to above thesoftening point. Surface forces here, again, generate a smooth layersurface and the synthetic layer can again be used for recording.

Because of the possibilities of carrying out record/- read out/erasecycles and of effecting the development in only a short time, i.e., inthe order of seconds, at the recording location, thermoplastic materialsare particu larly significant where holographic storage is concerned.Also, the photosensitivity of the thermoplastic materials of this typeis at least as good as that of conventional photographic emulsions.

In executing holographic recording cycles of this kind in thermoplasticmaterials, however, it is only possible in practice to carry out alimited number of such cycles because the signal to noise ratio, andtherefore the efficiency, becomes poor from cycle to cycle andultimately ceases to satisfy certain minimum requirements.

The reduction in the signal to noise ratio, and therefore the reductionin image quality and in efficiency, may be traced to several factors.For example, the properties of the thermoplastic material deterioratesbecause of the thermaloading applied during development and erasephases. Also, deterioration occurs because of the residual charges leftover from preceding recording cycles. Again, the electrostatic chargesresult in the absorption of dust at the layer surface, which dust,during the development and erase processes, when the layer softens,becomes embedded in the layer. The embedded dust particles act ascondensation nuclei in the neighborhood of which the layer breaks upcompletely after a certain number of cycles.

Furthermore, with increasing number of cycles, increased wrinklingoccurs in the layer. This is also due to the fact that the surfacetension at the time of erasing is insufficient to restore a smoothsurface from the relief structure of the previously recorded hologram.

SUMMARY OF THE INVENTION Consequently, the object of the presentinvention is to provide a method for the cyclic, reversible, holographicrecording of a thermoplastic material of the type initially described,in which with an increasing number of cycles the signal to noise ratiois substantially better and therefore the image quality is substantiallybetter than is the case in previously. known meth ods.

To achieve this object, the present invention is based on the proposalthat with an increasing number of re cording cycles, the corona chargevoltage required for sensitizing is increased and that the thermoplasticmaterial is thus placed in a progressively higher positive or negativepotentials.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantagesof the invention, its organization, construction and operation will bebest understood from the following detailed description of a preferredembodiment of the invention taken in conjunction with the single FIGUREwhich illustrates a section through a recording layer suitable forpracticing the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, alayer carrier 3, which may, for example, comprise a transparentsynthetic foil, is carried upon a grounded substrate 4. At its freesurface, the layer carrier 3 supports a layer 2 which may comprise, forexample, a mixture of a photoconductive and a thermoplastic material.

A corona charge device 1 is provided above the layer 2 and rendersitpossible to produce at the free surface of the layer 2 a charge, forexample a positive charge. This charge sensitizes the thermoplasticmaterial, the sensitizing being carried out in darkness. Subsequently,the thermoplastic material 2 is holographically exposed and a chargeprofile corresponding to the optical interference pattern resultswherein the charges migrate to the' exposed locations. The developmentof the hologram is carried out thereafter by heating. By means ofelectrostatic forces, deformation of the free surface of the layer 2 isproduced, this deformation being determined by the illumination energy.By rapid cooling, a stable hologram is created which, by heating to atemperature higher than the developing temperature, can be erased again.

With the formation of the holographic relief image at the free surfaceof the layer 2, the magnitude of the electrostatic surface tension playsa significant part. In accordance with the method of the presentinvention, the layer 2 is sensitized prior to the first record/erasecycle using a relatively low corona charge voltage. This charge voltageis correspondingly increased as the number of cycles progresses, so thatthe thermoplastic material is placed at progressively higher positive ornegative potentials.

Recording as disclosed above produces the result that the signal tonoise ratio, i.e., the relative image quality, drops off substantiallymore slowly during read out, as the number of cycles progresses, so thatconsequently substantially more record/erase cycles are possible.

Although I have described my invention by reference to a particularembodiment thereof, many changes and modifications of the invention maybecome apparent to those skilled in the art without departing from thespirit and scope of my invention. I therefore intend to include withinthe patent warranted hereon all such changes and modifications as mayreasonably and properly be included within the scope of my contributionto the art.

I claim:

1. A method of carrying out cyclic, reversible, holographic recording ina thermoplastic material, comprising the steps of: creating a coronacharge with a corona discharge voltage; sensitizing thermoplasticmaterial with the corona charge prior to each new recording cycle;progressively increasing the corona discharge volt age to increase thecorona charge for sensitizing prior to each new recording cycle;holographically exposing the sensitized thermoplastic material; heatingthe exposed material to produce a surface relief; and fixing the surfacerelief by cooling the thermoplastic material.

1. A method of carrying out cyclic, reversible, holographic recording ina thermoplastic material, comprising the steps of: creating a coronacharge with a corona discharge voltage; sensitizing thermoplasticmaterial with the corona charge prior to each new recording cycle;progressively increasing the corona discharge voltage to increase thecorona charge for sensitizing prior to each new recording cycle;holographically exposing the sensitized thermoplastic material; heatingthe exposed material to produce a surface relief; and fixing the surfacerelief by cooling the thermoplastic material.